Growing Things: Prevent tomato blight and potato scab

Could tomato blight and potato scab issues be related to compost that I added? It was purchased from a gardening company and was said to be organic. Do you have any suggestions on how I can avoid these issues next year?

Q: I have a fairly large vegetable garden on a rural acreage. This year I had a load of compost added to the soil. We have high sodium in our well water so I water by hand from rain barrels, but depend mainly on Mother Nature for watering.

I planted a large number of tomato plants (50) that I had grown from seed — San Marzano, Mortgage Lifter, Mixed Heritage, and some seed saved from a hothouse tomato grown locally. I rotate the location every year, and I watered the tomatoes between rainy periods.

My tomatoes were doing well and produced a great deal of fruit. Near the end of August, I noticed that some of the plants seemed to be dying and the fruit had begun to spoil on the vine. It looks a lot like the images on Google of late-season blight, so I had to discard about half the crop and picked off what I could. The majority of the green tomatoes are ripening nicely in the basement.

I also planted some potatoes around the perimeter of the garden, in a different location from last year, and I depended on Mother Nature to water them. I don’t believe our potatoes bloomed this year, although we were away for a week at the beginning of July and may have missed it. They produced a few potatoes, but nothing near the crop I normally get. The tops died very early in the season and the white potatoes are scabby. The red potatoes are on the small side, but don’t seem to have scab.

Could the issues with the tomatoes and potatoes be related to the compost that I added? It was purchased from a gardening company and was said to be organic. Do you have any suggestions on how I can avoid these issues next year?

A: I’m impressed with the effort you go to with your garden. You seem to be doing all the right things — even doing crop rotation. Some of what you ask is difficult to be specific about the cause, but we can address some of your issues directly.

The late-season blight has nothing to do with the compost you added. Unfortunately, there is no silver bullet when it comes to preventing this disease. The one thing I would recommend is looking for varieties that are said to be resistant to the blight. Varieties that mature earlier work best as well. I have grown the varieties you mentioned and had some blight issues with them myself.

Give the plants lots of space to allow for air circulation and avoid watering from above. Late-season blight is a fungal problem, and some gardeners cover their tomatoes or grow them under eaves to avoid this problem. You can also try a biofungicide called Serenade. There have been some reports of success using this eco-friendly product on late blight.

Could the scab come from the compost? It could, but most garden centres that sell compost are pretty careful about the product they sell. To me it sounds like the red potatoes might have been a variety that is more resistant to scab than the white potatoes were. Common scab of potatoes is a disease of bacterial origin. Scab is usually brought into the soil by infected tubers, and will survive indefinitely in the soil.

Scab is more of a cosmetic disease. It does not affect production, only the appearance of the potatoes is affected. While it is possible to adjust the soil pH to inhibit the disease, altering the pH could also change the optimum growing conditions for the potatoes and affect yields. It can also be costly to change the pH. While eliminating scab is very difficult you can reduce its occurrence and severity in several ways:

– Try to plant scab-free tubers or seeds.

– Rotate the crops in the affected area for three or four years between potato crops.

– Keep the potatoes well-watered during the formation of the tubers (four to six weeks after planting).

– Plant varieties that are more resistant to scab, such as Norland (red) and Kennebec (white).

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